The present invention relates to a frequency modulation system capable of stably frequency-modulating output frequencies of a variable frequency oscillator.
A multi-channel FM transmitter includes a variable frequency oscillator. The variable frequency oscillator must generate the center frequencies of the respective channel FM transmission waves so that the oscillation frequency of the variable frequency oscillator must be controlled in a relatively wider range as the number of channels increases, Generally, it is difficult to stably frequency-modulate such a wide range of fruencies. In a conventional FM transmitter, the output frequency of the variable frequency oscillator is frequency-modulated by means of a FM modulator and then the resulted FM wave is multiplied until the desired modulation index is obtained. This conventional scheme, however, is defect in that it is difficult to frequency-modulate with uniform stability such a wide range of frequencies and the occurrence of spurious components is inevitable at the frequency division stage.
Accordingly, an object of the present invention is to provide a frequency modulation system in which, even if a variable frequency oscillator produces a wide range of frequencies, the output frequencies may be easily and stably frequency-modulated without producing the sprious components or frequencies.